1. Field of the Invention
This invention relates generally to precision motor speed regulation circuitry and, more specifically, to a system for optimizing power consumption and speed regulation in a low-inertia capstan motor.
2. Description of the Related Art
Magnetic tape drives provide a fast and efficient means for storing computer data in large blocks, such as for backing-up the contents of hard disk storage devices. These tape drives, with tape cartridges using 0.25 inch, 8 mm, and 4 mm tape widths, are available for storing data in capacity ranges that match the requirements of small to medium computer systems. For example, commercially available devices can store over 1500 megabytes of digital data on quarter-inch (6.35 mm) magnetic tape in standard cartridges. Such drives can be packaged in the same slot that provides for an industry-standard half-height floppy disk drive.
Streaming magnetic tape drives, also called streamers, are constant-speed tape transports intended for back-up storage of the contents of hard disk drives. Streaming drives usually record data bidirectionally, laying down as many parallel tracks as permitted by tape head technology. Current tape head technology permits about 30 parallel tracks on quarter-inch tape (.about.5 tracks/mm).
Practitioners in the art have introduced the use of low-inertia motors in tape drives as a technique to reduce power consumption during the motor start-up sequence. These motors are smaller and more economical with power but have low inertia compared to the load. This relatively low inertia causes the motor acceleration to vary proportionally with motor load. Thus, the time required for the low-inertia motor to start up and reach operating speed also varies proportionally with the motor load.
In accordance with specifications generally established by groups such as the American National Standards Institute (ANSI) and the Quarter-Inch Cartridge Committee (QIC), the load on a capstan motor resulting from a QIC tape cartridge can vary from perhaps 0.25 newtons to 1.5 newtons. Therefore, tape cartridge drag varies from cartridge to cartridge over a 6 to 1 range. Also, drag forces can change with age and use of any particular cartridge, depending on frequency of use and operating temperatures. Moreover, the drag imposed on a capstan motor varies directly with tape speed.
Because the cartridge load varies over a 600% range, the motor startup time may vary also by 600% from cartridge to cartridge. This makes it difficult for the tape drive designer to meet the worst-case "up to speed time delay" of 300 milliseconds established in the QIC industry standard equipment specification.
Moreover, the industry standard tape cartridge is designed to withstand a maximum tape acceleration of only 1500 ips/sec (38 m/sec.sup.2) . Thus, if a drive motor current limit is set to permit a high-drag cartridge to speed up within the worst-case time limit, a low-drag cartridge inserted in the same drive may be destroyed by the startup torque and acceleration of several times the necessary amount. On the other hand, if the motor current limit is set to control the acceleration of a low-drag cartridge, a high-drag cartridge in the same drive will require several times the specified worst-case speed-up time.
Practitioners in the art have suggested several solutions to this problem of optimizing startup-delay and tape acceleration with low-inertia capstan motors employed in streaming tape drives. One class of existing startup techniques applies a fixed drive to the motor in the hope that it will turn before the drive current exceeds the motor limit. Another such class merely starts with half-power, going to full power if the motor does not turn immediately.
Speed ramp-up control techniques in the prior art teach the use of a fixed drive current ramp from an initial value to a full speed value without adjustments for differences in motor responsiveness. Also, the effectiveness of motor speed regulation is limited to a 20-30% range of full speed in the present art.
In U.S. Pat. No. 4,963,810, A. D. Rojas et al disclose a variable load motor drive control circuit for making low inertia motors useful under circumstances where the load on the motor during startup acceleration may vary. Rojas et al employ a ramped drive current as opposed to the earlier, more conventional constant current limit signal. They first provide an initial fixed current to overcome the static friction of the motor and then ramp the current upward at a predetermined rate expected to bring the motor to operating speed without exceeding the desired acceleration. However, Rojas et al neither teach nor suggest means for adjusting the motor drive responsive to actual motor velocity. They merely apply the predetermined drive signal to the motor and wait for a predetermined time, without checking to see if the motor actually starts. Also, Rojas et al do not vary the initial drive level to accommodate differences in starting friction from motor to motor and cartridge to cartridge. Thus, their method is not adaptable to optimizing power consumption by reducing drive current to lightly loaded motors or increasing drive current to force a sticky motor to move within a desired time limit.
In U.S. Pat. No. 4,943,907, Kurt E. Godwin discloses a speed controller for a multi-speed peripheral disk or tape drive that automatically selects a drive speed from among a set of predetermined drive speeds. Godwin neither teaches nor considers drive level regulation responsive to motor load but merely teaches an automatic selection process involving an automated discovery of the maximum common data rate specification between a Central Processing Unit (CPU), its peripheral drive controller and a disk or tape drive.
Thus, there is still a clearly felt need in the art for a tape drive capstan motor controller that optimizes the motor startup power consumption and motor startup time delay for all combinations of motor inertia and tape cartridge drag. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.